The sound of clinking glass fills the room as a monk inspects one of hundreds of beer bottles passing along the production line at Andechs monastery in Bavaria.

Benedictine monks have been making beer for hundreds of years here and following Germany's oldest and most revered brewing tradition - the 'Reinheitsgebot' purity law that says beer must be made with just a select few ingredients.

"Germany's purity laws say that in beer only three ingredients are allowed to be present -- hops, malt and water," Andechs monastery spokesman Martin Glaab said. "At that time, yeast wasn't yet known, but yeast of course also belongs there too."

This year marks 500 years since the Reinheitsgebot was decreed in the southern state of Bavaria. While no longer actual law, it is still regarded as an important tradition and for many brewers a guideline on how German beer ought to be made.

At Andechs, whose hilltop church draws pilgrims, more than 100,000 hectoliters of beer are produced each year. Tourists come from afar to taste such brews as the Doppelbock Dunkel and Weissbier Hell, which are also exported worldwide.

While Andechs takes pride in following tradition, some Germans brewers are critical of the Reinheitsgebot, saying it does not allow enough range to experiment with new recipes.

"We have different kinds of hops, different kinds of malt ... that we can combine with each other," Glaab said.

"Theoretically it is possible to brew more than one million different kinds of beer, if you really want to extend the purity law and that, until today, hasn't happened."

Beer fans across Germany will mark the 500th anniversary of the purity law with a variety of celebrations. In Bavaria, beer will flow from a fountain in the town of Ingolstadt and exhibitions and festivals are also planned.

Andechs monastery plans to mark the anniversary with the release of a celebratory beer next month.